In the process for manufacturing an Si transistor, there has conventionally been used a step of washing a substrate to be treated with an HF solution for the removal of the naturally-occurring oxide film present on the surface of the substrate, as a preliminary step for forming salicide composed of Ni and Co for ensuring the contacts between the sources or drains of the transistor and the distributing wires or interconnections thereof and also as a preliminary step for ensuring the contact between the gate poly-Si films and the distributing wires or interconnections of the transistor. However, such an HF solution cannot satisfactorily enter into the fine holes of the device as the latter undergoes gradual shrinkage with the elapse of time and accordingly, the naturally-occurring oxide film cannot sufficiently be removed.
To solve the aforementioned problems, a radical etching technique (CDT) in a gaseous phase has recently been used, which makes use of NFH radicals (NFxHy radicals) such as NF2H or NFH2 radicals. When carrying out the removal of an SiO2 film through etching procedures with the use of NFH radicals, (NH4)2SiF6 is formed as a residue product. The removal of this residue product has in general been carried out through evaporation, while heating the product to a temperature on the order of 200° C. This method for the removal of (NH4)2SiF6 as the residue product makes the most use of the characteristic properties peculiar to the same such that it can be vaporized at a temperature of about 120° C. To form NFH radicals, there has been used such a method which comprises the steps of decomposing N2 gas and H2 gas or NH3 gas by the irradiation of the gas with the plasma generated by the application of microwaves (hereunder referred to as “μ waves”) to thus form H radicals; introducing the resulting H radicals into a vacuum chamber; and then allowing the H radicals to undergo a reaction with NF3 separately introduced into the vacuum chamber (see, for instance, Patent Document 1 as will be specified below). The generation of a plasma through the use of μ waves has been carried out by introducing N2 gas and H2 gas or NH3 gas into a quartz tube or a sapphire tube and then irradiating the gas mixture with μ waves. However, the lifetime of the H radicals thus produced is quite short and a problem correspondingly arises such that it is quite difficult to transport the H radicals to a vacuum chamber without any deactivation of the same.
Conventionally, the formation of NFxHy radicals has been accelerated by controlling the direct formation of HF while taking care not to cause any direct irradiation of NF3 with plasma or ions and the NFxHy radicals thus generated have been used for the etching of SiO2. In this case, the related reaction formulas (1) to (3) will be those described below (according to the NFH etching mode):SiO2+6NFH2+6H2→(NH4)2SiF6+2H2O+4NH3  (1)SiO2+NFH+5HF+NH3+H2+H→(NH4)2SiF6+2H2O  (2)SiO2+2NH4++2HF2−+2HF→(NH4)2SiF6+2H2O  (3)
However, if HF radicals directly reach a layer of SiO2, the direct etching of the SiO2 layer with HF proceeds as will be illustrated by the following reaction formulas (4) and (5), in addition to the foregoing three reactions (according to the HF etching mode):SiO2+4HF2H2O+SiF4:SiF4+2HFH2SiF6   (4)SiO2+6HF2H2O+H2SiF6   (5)
In respect of the reactions according to the foregoing reaction formulas (4) and (5), the reactions according to the following reaction formulas (6) and (7) take place simultaneous with the reactions represented by the formulas (4) and (5), unlike the foregoing reactions represented by the reaction formulas (1) to (3), in which NFH radicals are involved, the reactions according to the formulas (6) and (7) are correspondingly accompanied by the generation of a large quantity of F and, in this case, the etching of the SiO2 layer would proceed according to the F etching mode. For this reason, this would arise a problem such that the SiNx film on a semiconductor device is likewise etched in some cases.N+NF3→N═NF3→N═NF2+F  (6)N+NF3+N→N═NF2+NF:NF+N→N2+F  (7)
However, this reaction, in which HF is involved, is advantageous in that the reaction is not accompanied by any self-stopping effect or it is never terminated autonomously and that the etching rate thereof is very high unlike the reactions in which NFxHy radicals are involved. Therefore, a thick film can be etched at a high speed if it is not necessary to take into consideration the selectivity ratio with respect to the etching of a thermally deposited-nitride film (SiN film).